Abstract : Whereas the red cells recover from the actual congelation of all the water freezable at -3C, the neutrophils are already injured when only a fraction of the water freezable at -1.5C is congealed. Electron microscope studies of freeze-dried or freeze-subsituted suspensions of leucocytes indicated that freezing at rates varying from 30 to 2000 degrees C per second results always in the formation of intracellular and intranuclear ice in neutrophils. A study by concentrations of solutions of the cryoprotectants: ethylene glycol, glycerol and polyvinyl pyrrolidone, undergo homogeneous nucleation throws some light on the conditions under which ice nuclei may originate in cryoprotected biological material. (Author)